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Optimized Flexible Circuit Impedance Measurement

Publishing Venue

IBM

Abstract

Characteristic impedance control on flexible circuits is a requirement for successful implementation in high speed applications. However, due to manufacturing, process and cost limitations associated with flexible environments, standard impedance control processes can affect yield and piece cost. This document describes a method by which entire panels of flexible circuits can be screened for proper characteristic impedance without adversely affecting yield, parts-per-panel and cost.

Country

Undisclosed

Language

English (United States)

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Optimized Flexible Circuit Impedance Measurement

Given increasing bus speeds on flexible circuits and increased industry use of this
technology, impedance control on such circuits must be maintained. Differing from rigid
card applications, time domain reflectometry (TDR) test coupons that can be used to
measure the characteristic impedance are not typically placed on flex circuit parts. This
is due mostly to space constraints typically placed on flexible circuits and no real estate
remains available for test coupons. Obviously, this makes impedance testing difficult, as
measurements are difficult or impossible, depending on the length of available signal
traces, to make on the actual part. Previous solutions to this problem are to place test
coupons on the parts panel, outside of the actual part outline. The drawbacks to that
solution are:

Typically, there is not enough real estate to add coupons on a panel without

1.

2.

3.

4.

affecting the "number-up" (parts per panel) and, therefore, cost.

There exists process variations across a panel and therefore multiple test coupons

must be strategically located across a panel to account for impedance variations
inherently present from one part to another.

Requiring several different test coupons to be measured increases piece price.

Uncertainty remains as to whether adequate impedance correlation exists between

test coupon impedance and that of the signals on a part several inches away from
this test coupon.

This invention solves several problems associated with the inability to place TDR
test coupons on flexible circuits. This idea eliminates the need for multiple test coupons,
outside actual part outlines, placed across a panel. The invention also reduces
measurement time, cost and real estate consumed by test circuits. Additionally, the idea
increases correlation between test coupon impedance and that of an actual part. The
invention utilizes an impedance test line integrated with the base panel that is used to
carry the actual flexible circuits (see Figure 1).

I.The Problem:

It is critical to constrain and control characteristic impedance on flexible circuit
designs. However, space constraints do not allow for impedance test coupons to be
placed on the actual part. This creates the need for:
external coupons to be placed on the panel, resulting in consumption of panel real

estate and increased piece cost.
measurements to be made on the actual flex circuits. However, most flex circuit

trace lengths are not sufficiently long to accommodate accurate impedance
measurements without expensive TDR equipment with extremely low rise times.